In high-performance liquid chromatography, samples to be examined are fed into a high-pressure stream of liquid in order to be delivered to a chromatography column for analysis. For this purpose, a precisely defined amount of the sample is made available by being aspirated into a line (suction line), which forms part of what is called a sample loop. The line contains a solvent and is connected at a first end to a cylinder in which some of the solvent can be aspirated from the line by means of a piston. The other, second end of the line is immersed in a sample vessel prior to the aspiration. The volume of the solvent taken up on aspiration by the cylinder at the first end of the line corresponds to the volume of the sample aspirated at the second end of the line. By precise positioning of the piston, a precisely defined sample volume can thus be taken up into the line or the sample loop. After aspiration of the sample, further solvent can, if necessary, also be aspirated at the second end of the line, such that the sample is present as a “plug” in the line, enclosed on both sides by solvent.
Thereafter, the line is connected with its second end to a port through which the sample is intended to be conveyed out of the sample loop into a chromatography column. For this purpose, either the cylinder is integrated, together with the attached suction line, into the circulation of the solvent, or its content is conveyed back into the suction line via a branch line.
In the following, “sample loop” is to be understood as at least the content of said line and of the volume aspirated into the cylinder.
At first, the sample is generally at ambient pressure during the aspiration. However, the chromatography method is performed at very high pressure (>100 MPa), for which purpose a special pump is provided which forces the solvent as carrier medium through the chromatography column. To be able to feed the sample from the sample loop into the high-pressure column, the sample loop is integrated via a suitable valve into the delivery path after the aspiration and thereby conveyed to the chromatography column. So that the solvent provided in the sample loop volume can be brought together with the sample to the necessary high system pressure (thereby avoiding sharp fluctuations in pressure when the sample loop is switched in), the volume in the cylinder is compressed with the aid of the piston to approximately the expected operating pressure. The switching of the valve for injecting the sample into the chromatography column then takes place without pressure fluctuations and permits substantially stationary operation of the plant. To convey the sample out of the sample loop, it is possible, by switching the valve, to integrate the cylinder into the delivery path such that it flows through the latter. Alternatively, the piston can also drive the compressed volume actively into the line. Details of this method can be found in particular in DE 10 2008 006 266 B4, the disclosure of which is intended to be incorporated here in full.
If the sample loop is adapted to the high system pressure before being integrated by valve switching into the delivery path to the chromatography column, this affords in particular the advantages that the column does not experience any abrupt and possibly damaging pressure surges, and that the retention times remain constant (reproducibility). This is achieved if the sample loop is already brought to system pressure before switching on.
In this method, the piston of the sampler is on the one hand moved in order to aspirate a very precisely defined sample volume (of the order of microliters), while on the other hand it is subjected to very strong forces during the pre-compression of the sample loop to the operating pressure. The drive moving the piston thus has to meet very strict requirements both as regards the precision of the piston stroke and also as regards the strength of its components during the pre-compression. On account of the high forces during the pre-compression, larger and more robust components are needed, but this also leads to poorer reproducibility in the provision of the precise sample volume. Moreover, the spindle of the drive, designed for the high-precision positioning of the piston, is subjected to a considerable load during the pre-compression, which results in wear, abrasion and poorer precision. It has not hitherto been possible to provide different configurations of the drive that would ensure both the necessary precision and also the required strength.